The present invention relates to a high frequency technology applicable to a microwave and a millimeter wave, and more specifically to a high frequency circuit substrate capable of relaxing a positional precision when circuit substrates are electromagnetically coupled, or of easily ensuring a required positional precision while elevating a close contact between electromagnetic coupling parts, and a connecting method therefor.
In the prior art high frequency circuit, when a millimeter “multi-chip module” (abbreviated “MCM”) and an antenna are connected, a coaxial cable or a waveguide (in narrow meaning) is used in many cases. In a relatively large scaled module, similar means is used for interconnecting a high frequency circuit substrate to another high frequency circuit substrate. Therefore, the high frequency circuit substrate and the antenna are required to have a waveguide conversion structure or a coaxial connector.
In the prior art, on the other hand, various methods which uses neither the coaxial cable nor the waveguide, have been proposed to realize a microminiaturization and a low cost. For example, there has been proposed a prior connecting technology for interconnecting between a microstrip line on the high frequency circuit substrate and a microstrip line of a planner antenna by a wiring bonding (See Y. Hirachi et al, “A Cost-Effective RF-Module with Built-in Patch Antenna for Millimeter-Wireless Systems”, 29th European Microwave Conference, Digest, Vol. 3, pp.347-350, 1999). Alternatively, there has been proposed another prior connecting technology of mounting an integrated circuit on a rear surface of an antenna board in a flip chip manner, and connecting the integrated circuit to the antenna by means of a via hole or by action of an electromagnetic coupling (See G. Baumann et al, “51 GHz Frontend with Flip Chip and Wire Bond Interconnections from GaAs MMICs to a Planar Patch Antenna”, 1995 IEEE MTT-S International Microwave Symposium, Digest, pp.1639-1642, or Y. Amano et al, “Multi-Layered Substrates for Wireless Communication Modules at 60 GHz”, 29th European Microwave Conference, Digest, Vol. 2, pp.301-304, 1999).
However, in the prior technology using the wire bonding, particularly when an array antenna of a high gain is connected, another problem is encountered in that a feeder line loss becomes large. On the other hand, a prior technology for mounting an integrated circuit on a relatively large board such as an antenna board is difficult to ensure a planar pattern precision or planarity, and therefore, can be applied to a highly precise mounting technology such as a flip chip method and a BGA (Ball Grid Array) method.
In order to overcome the above mentioned problems, for example, Japanese Patent Application Pre-examination Publication No. JP-A-09-237867 proposes a small-sized high frequency package structure which can be manufactured in a mass production and which can reduce the loss attributable to a connection between an antenna circuit substrate and a high frequency device circuit substrate.
Referring to FIG. 14, there is shown a diagrammatic sectional view of a main part of a prior art high frequency circuit substrate. A first high frequency circuit substrate 1 includes a first dielectric material layer 3, a first conductor layer 41 having a slot 4 formed therein, a second dielectric material layer 5 and a second conductor layer 24 so that a high frequency transmission line 6 is formed of the second conductor layer 24. A second high frequency circuit substrate 2, which has an antenna (not shown), is layered over the first high frequency circuit substrate 1 to be connected to the first high frequency circuit substrate 1. This second high frequency circuit substrate 2 includes a third dielectric material layer 9 similar to the first dielectric material layer 3, a third conductor layer 51 having a slot 10 formed therein, a fourth dielectric material layer 11 and a fourth conductor layer 52. A high frequency transmission line 12 is formed on a surface of the second high frequency circuit substrate 2 facing the first high frequency circuit substrate 1. On the opposite surface of the second high frequency circuit substrate 2, an element 34 of the antenna is formed of a conductor pattern.
With this construction, a radio signal received by the antenna element 34 is guided by an electromagnetic coupling through the slot 10 formed in the second high frequency circuit substrate 2, to the high frequency transmission line 12, and further, by an electromagnetic coupling through the slot 4 formed in the first high frequency circuit substrate 1, to the high frequency transmission line 6.
However, an actual problem is a precision in lamination of the first high frequency circuit substrate. In this connection, a prior packaging technology having this construction and constituted by use of lamination, is proposed by, for example, Kooriyama et al, in Technical Bulletin ED-99-214/MW99-146, pp35-42, Society of Electronics, Information and Communication Engineers (of Japan). However, this prior art involves a severe problem in which a high precision is required in positioning or alignment between the slot 4 and the high frequency transmission line 12 formed on the second high frequency circuit substrate 2 shown in FIG. 14.
For example, assuming that an operating frequency is 60 GHz, a dielectric constant of the dielectric material is 10, and the transmission line is a microstrip line, FIG. 13 shows an increment of connection loss caused by a positional deviation (in a signal direction) between the slit and the microstrip line. Referring to FIG. 13, if the increment of connection loss is defined as 0.5 dB, an allowable positional deviation becomes ±0.15 mm.
As a technology which can meet with this precision and which is suitable to mass production, there is a method of forming the first and second high frequency circuit substrates together by using a ceramic material. In this method, there is a restrictive condition that the same quality of material must be used for the first high frequency circuit substrate 1 and the second high frequency circuit substrate 2. In addition, when substrates having different dielectric constants are used, when an inner layer part is included, when a cavity structure is involved, or when the substrate is relatively large, it becomes difficult to obtain a device size and a device constant as designed, by use of sintering, with the result that the yield of production lowers.
In order to overcome the above mentioned problem, JP-A-09-237867 proposes a method for bonding independently formed high frequency circuit substrates by use of an alloy such as AuSn, soldering or an epoxy resin. However, in this prior art for bonding the independently formed high frequency circuit substrates, it is required to maintain the circuit substrates in a close contact condition so that no electromagnetic leakage occurs. However, when a conventional ceramic formation technology is used, a bowing occurs on the order of 0.03 mm to 0.06 mm per 1 cm square. Therefore, in a large substrate area such as in a large scaled multi-chip module, it is difficult to bring the connecting parts in close contact with each other so that the respective grounds of the substrates cannot be effectively connected to each other.
Furthermore, this prior art includes another problem in which it is difficult to meet with the requirement of the positional precision. For example, when the positional precision of ±0.15 mm is required, a mounter having a high degree of positioning precision is required. In order to obtain the high degree of positioning precision, it may be considered to utilize a BGA mounting technology using a self-alignment process. When the bowing of the substrate is large, it is difficult to apply this technology.